Beginning in 2012, one of the authors (S. P.) characterized a series of zirconia-based coatings. That investigation was motivated by zirconia (ZrO\(_2\) ) being a high-index material (\(n \ge 2\)  at 1064 nm, like tantala) possessing a similar molecular structure and physical parameters to silica. However, unlike silica, when treated at high temperatures, zirconia will crystallize unless stabilized. Tewg et al. [1]} showed that the addition of zirconia to tantala thin films allowed for a higher maximum heating temperature, before the onset of crystallization. Post-deposition heat treatment, referred to as annealing hereafter, is a standard procedure to decrease the internal stress, the optical absorption and the mechanical loss of coatings [2]}. As a general empirical rule, the higher the annealing peak temperature, the lower the resulting coating mechanical loss. In the work of Tewg et al., the annealing had a duration of 10 minutes, while typical annealing times for HR coatings used in GW detectors are of the order of 10 hours [2]}. Indeed, higher annealing temperatures (up to 900 \(^\circ \) C) and duration would be beneficial also for the silica layers, decreasing their mechanical loss [4]}, [5]} as a result of structural relaxation, leading to HR coatings with overall lower loss angle and hence lower thermal noise.
